A great and growing volume of freight is shipped around the world in standard shipping containers. Transshipment has become a critical function in freight handling. At each point of transfer from one transport means to another, from ship to shore in ports and harbours for example. There is usually a tremendous number of containers that must be unloaded, transferred to a temporary stack, and later loaded on to another ship, back onto the same ship or loaded instead onto another form of transport. Loading and unloading containers to and from a ship takes a great deal of time. The development of automated cranes has improved loading and unloading and made the productivity more predictable, and also eliminated many situations in which port workers have been exposed to danger and injury.
The technical demands of handling containers accurately are great. A container may be handled by a stationary crane or by crane moving on rails or moveable in any other way. Each crane has a lifting device usually incorporating a spreader of some kind that directly contacts a container, to grip it, lift it, lower it and release it. In this description the term spreader is used to denote a part of a lifting device that is in direct contact with a container. Spreaders are normally designed to handle more than one size of container, for example 20-40 ft or 20-40-45 ft long containers. The spreader is suspended from the boom of a crane from a moveable device known as a trolley, which moves along the boom of the crane, in a direction usually referred to as the X direction. The position of the trolley is measured and/or calculated during operations. The position of the spreader and the container underneath it may be monitored by use of a camera observing a light source or marker on the spreader. It is of great importance for accurate operation, and especially for automatically controlled operations, that the position of the container is accurately known during pick-up and during landing of a container.
Accuracy during pick-up is necessary for the spreader to grip the container properly at the first attempt. Accuracy during landing is important not only to land the container at the first attempt, but also because if an error in stacking containers one on top of each other that can lead to a cumulative error which may be unacceptable. When a 5-high stack of containers is not stable it presents a potential for containers to be damaged. An unstable stack also demands greater ground area and more clearance space around it for lifting operations.
Cranes may be operated automatically in many phases of each operation. However a crane operator is usually required to drive the crane to deal with situations that are not handled by existing automated operations. For example, when a container is lowered for landing there is often a torsional movement of the container, known as a skew. With a skew problem, when the long axis of the container swings around a vertical axis in a skew (torsional) direction, it can take many seconds, perhaps up to a minute, before the skew oscillations die down enough for the container to be lowered on to a truck, container or other target. The container cannot be landed accurately if it is not accurately lined up above the landing target. When unloading a ship with perhaps many hundreds of containers, the cumulative effect of unloading time lost due to skew oscillation is considerable. Manual adjustments may be made by the crane operator to cancel out a skew moment by steering the spreader against the moment or by operating auxiliary adjustment devices. However the effectiveness of manual intervention is operator dependent and does not reliably reduce the time lost to skew oscillation.
Application JP2001322796 entitled Vibration control device for a load, to Mitsubishi, describes a device suspending a conventional spreader fitted with four tension sensors to measure rope tension in the load ropes. A tension sensor is fitted to each lifting rope near a point where the rope is fixed, arranged so that there are two sensors on one side of the spreader and two on the other side. At the non-fixed end two main winding drums are arranged for lifting the load, to wind in or wind out, so as to lift, lower the container. A skew cylinder mechanism is arranged connected to sheaves arranged on each side near the winding drums so as to exert a greater tension force on the load ropes on one side of the spreader and a corresponding lesser tension on the load ropes on the other side of the spreader, so as to counteract an error in skew angle. Measurements of rope tension on each end of the container are compared. A skew angle θ (theta) is calculated from the measurements of rope tension combined with calculations of a distance between trolley and spreader based on measurements of the rotational frequency and angle of rotation of the winding drums. An online automatic translation of the description of JP2001322796 explains that use of tension sensors provides a way to detect skew which may be better than more expensive optical means. However, the described device depends on comparable measurements of tension for each end of the container which makes the device liable to error in cases where weight distribution inside the container is uneven and one end of the container is heavier than the other. It is also somewhat problematic to rely on tension sensors normally of the load cell type. These are usually large and heavy analogue devices that require calibration at frequent intervals to maintain the level of relative accuracy such load cells can provide. Similarly, the abstract of JP10017268, to Mitsui, entitled Skew swing preventive method and device of crane suspending cargo, describes a device that includes the use of tension sensors in the load ropes. Optical detection means for determining a skew angle are also described. This device or system uses measurement of tensile forces in the lifting ropes, together with measurement of angular velocity and skew angle by means of a CCD camera, to find or calculate an angular skew error and a skew oscillation period. A natural oscillation period is calculated from a calculated moment of inertia by a computer for the hanging container. Rope tension is then applied to one or other end of a loading rope by means of an actuator arranged at each end of each loading rope. The driving force required by the actuator is reduced by the directional changes of the loading ropes and addition of extra sheaves, and tension balancing sheaves, so that the load of the hanging container does not act directly on the actuators. A computer is used to apply counter tension by means of actuators mounted on both sides of the trolley until the skew error is found to be zero. However, like JP2001322796 (above) the described system relies principally on measurements of rope tension. Rope tension is also influenced by forces other than a diagonal or skew movement of the container, including forces due to uneven weight distribution in the container. Rope tension is more of measure of some of the forces acting on a container rather than a direct measure of container position. Accurate measurement of angular velocity of rotation using a camera may be somewhat difficult in practice, especially when the angular/rotational velocity of a container varies, or is combined with other non-skew movements. Accuracy of load cells as tension sensors tends depends on calibration at intervals. A disadvantage with this approach is that although calculations may be carried out to compensate for skew angle error due to stretching of the ropes under load, spreader-load calculations based on a dynamically changing rope tension may include errors that are hard to predict and thus difficult to compensate for.
As well as skew deflection in which the long orthogonal axis of the container rotates or oscillates, the short side of the container may be displaced or may oscillate, giving rise to a movement about the long orthogonal axis of the container, a movement called a list. This may be caused by inertia during acceleration, uneven winds etc, or uneven loading inside the container, or a combination. When the short axis of a container is deflected or rotated about the long axis in a list movement then one long edge of bottom of the container is lower than the other. When a container is listing the actual position of the bottom of the container may not be predicted accurately. A consequence of this is that the bottom of the low side of the container will touch down inaccurately, sometimes by up to 10-25 centimeters or so away from the intended target. Such inaccurate placement gives rise to an unstable or even dangerous stack when containers are stacked in piles of 5 high. It means that manual intervention by the crane operator is necessary to maneuver the container to solve the problem of inaccurate landing due to a list of the container.
There is a similar and third type of container deflection which can arise during loading or unloading in which one end of the long axis of the container may hang down lower than the other end, a movement, displacement or deflection called trim. A trim problem can occur for example when loads inside a container are unevenly distributed, so that when lifted, one end container tends to hangs down lower than the other. This type of error can also lead to inaccurate loading or stacking, as the position of the ends of a container with a trim error are not directly vertically underneath the spreader, and thus not accurately predicted. A trim error can also cause errors of position during landing and usually requires manual intervention by the crane operator to prevent causes error in placement of containers, for example on a truck and in the stacking of containers, for example in a yard or on a ship.